In the Mead et al copending U.S. patent application Ser. No. 660,652 filed Feb. 23, 1976 and entitled METHOD FOR AUTOMATIC FABRIC INSPECTION, assigned to the same assignee as the present invention, there is disclosed a basic method of fabric inspection by analysis of the diffraction pattern developed from passing a coherent light beam through fabric material. In accord with that method, the height and shapes of side lobes developed in various regions of the diffraction pattern are compared to given references representative of a "good" quality of fabric. A grade count can thus be assigned to any fabric being inspected.
In the Fomenko copending patent application Ser. No. 660,653, now U.S. Pat. No. 4,057,351, filed Feb. 23, 1976 and entitled COHERENT SCANNING SYSTEM FOR FABRIC INSPECTION, also assigned to the same assignee as the present invention, there is disclosed a scanning system enabling high speed automatic inspection of large fabric areas to be carried out by the method disclosed in the first-mentioned copending application. Basically, this scanning system includes a scanning mirror which, through various optical components, causes the coherent beam to scan across the width of the fabric from one edge to the other. A descanning mirror has directed towards it the beam as it passes through successive areas across the width of the fabric, the descanning mirror then directing the coherent beam to appropriate detector optics.
In the Fomenko copending patent application Ser. No. 662,955, now U.S. Pat. No. 4,070,114, filed Mar. 1, 1976, entitled DETECTOR OPTICS FOR USE IN FABRIC INSPECTION, again assigned to the same assignee as the present invention, there are disclosed details of a preferred detector optics system enabling the simultaneous and individual analysis of each of the various first order side lobes developed in the diffraction pattern. The detector optics makes use of a linear photo-diode array for each first order side lobe of light to be analyzed and towards this end, the detector optics includes an optical squeezing means such as a cylindrical lens for squeezing the light lobe into a focused array of light for accommodation within the linear photo diode array. This squeezing not only removes elongation of the lobe as a result of astigmatic conditions all as explained in the last mentioned copending patent application but further results in a light intensity distribution along the diodes of the array such that voltage signals can be developed by scanning the light lobe from one side to the other by means of the array which signals are functions of the changing light intensity along the array.
The analysis of a lobe of light by a linear photo diode array is known in the art. For example, one such type of linear diode array is produced by the Reticon Corporation of Mountain View, California and is referred to by the registered trademark RETICON. These devices are referred to as self-scanning arrays in that they will successively analyse a light lobe from one side of the lobe to the other by "scanning" of the same. Equipment utilizing such linear photo-diode arrays for analysis are often referred to as video systems.
In many applications, diffraction pattern analysis in particular, the "shape" of a video signal contains implicitly or explicitly the information necessary to decide among alternative actions. In the conventional video systems there are typically necessary 500 or more picture elements (pixels) per scan line. If all pixels must be used in a processing system to determine an action decision, the rate these decisions can be made is necessarily limited. If additionally the location of the pertinent information is not fixed along the scan line, additional processing must take place.
Systems for analyzing the output data from linear photo diode arrays are known. For example, such systems are used for pulse height analysis in the nuclear field. Essentially, such pulse height analyzers utilize an analog peak detect and hold circuit on the common video of the linear photo diode array. The analog peak detect and hold circuits are very difficult to design for very short duration pulses. Further, they generally employ capacitors which must be discharged before measuring the next pulse.
As a consequence of all of the foregoing, the necessary extremely rapid inspection of fabric material in accord with the teachings set forth in the heretofore mentioned copending applications cannot readily be carried out with the prior art systems.